When combining a gas spring with a seat plate of a chair, an operative connection is established between a manipulating lever provided at the seat plate and a control pin extending beyond the upper end of the gas spring through a locking adaptor unit. This locking adaptor unit has variable length, such as to be adapted to the distance between the engagement point of the manipulating lever and the upper end of the control pin. Such by length adjustment of the locking adapter unit, tolerances resulting from the manufacturing of the gas spring, the manufacturing of the seat plate and the assembling of the seat plate and the gas spring can be compensated for.